Technical Field
The present disclosure generally relates to electronic circuits and, more specifically, to contactless transmit-receive terminals capable of operating in a first so-called reader mode, where the terminal communicates with a distant electromagnetic transponder, and in a so-called card mode, where the terminal operates as an electromagnetic transponder with respect to another terminal.
Description of the Related Art
A contactless card reader emits a magnetic field towards an oscillating circuit of a card generally having no autonomous power supply. In the reader-to-card direction, the data are generally coded and transmitted in amplitude modulation of a carrier of excitation of an oscillating circuit of the reader. In the card (transponder) to reader direction, the card circuits modulate the load formed by the oscillating circuit of the card on the magnetic field of the reader, with a circuit generally called a retromodulation circuit, for example, by short-circuiting the antenna circuit by means of a switch.
A radio frequency terminal transmitting data by inductive coupling, capable of operating in card mode and in reader mode, is capable of generating the data frame coding and decoding according to different protocols. Near field communication circuits (NFC) are capable of operating in card mode and in reader mode. Such circuits are commonly used in portable telecommunication devices such as cell phones. Various standards set the communication protocols.
In card mode, the power supply of the circuits is generally induced by the magnetic field generated by the reader, which forms the communication channel between the card and the reader. The alternating current (A.C.) signal exciting the oscillating circuit of the reader then forms a remote-supply carrier for the card.
In reader mode, the terminal is powered (for example, by a battery or by a connection to the power distribution system) to emit the magnetic field towards a card.
In dedicated devices, the antenna (or oscillating circuit inductance) of the reader is often tuned to the transmit frequency (for example, on the order of 13.56 MHz for ISO standard 14443) and matched to the impedance of the A.C. signal generator, while the antenna (inductance of the resonant circuit) of a card is often tuned to the operating frequency of the system (for example, 13.56 MHz).
If the reader antenna is tuned, but mismatched, the transmission is not optimal. Further, the reflected wave due to the mismatching disturbs the detection of the signal retromodulated by the card. Conversely, if the card antenna is matched but off-tune, the power recovery is not optimized.
As a result, antenna circuits are generally not compatible with an operation in card mode and in reader mode. In particular, it is difficult to use a single antenna to design a device which can operate both in reader mode and in card mode.
Another issue is linked to the need to associate filters against electromagnetic disturbances (electromagnetic interference, or EMI, filters) to the transmission circuit. Such filters are generally interposed between the electronic circuits of the transceiver device and the antenna.
U.S. Pat. No. 7,665,664 describes a reader by inductive coupling equipped with means for extracting a power supply voltage in a card mode operation by extracting the electric power directly in the antenna circuit of the reader without using a dedicated antenna coil. In this circuit, EMI-type low-pass filters are incorporated into the antenna circuit and are electrically interposed between terminals of the antenna circuit intended to be connected to a transmit circuit and series capacitors connected across the antenna. The architecture is of differential type and the antenna includes a grounded midpoint, two capacitors in parallel between each terminal of the antenna and the ground taking part in the tuning of the oscillating circuit. In card mode, the input terminals of the antenna circuit are short-circuited by a rectifying bridge. The capacitive elements taking part in the oscillating circuit as well as those in series with the low-pass filters then generate disturbances, which result in a detuning of the antenna in card mode, a loss of amplitude of the received signal, and in addition or alternatively, a deformation of the received wave.